The demand for high-speed digital communications services, such as data, video, and high-definition TV, is growing. To accommodate these services transmission systems that operate in the multigigabit per second range are being developed with technologists concentrating on developing optical transmission systems because of their large bandwidth capabilities. Such high speed optical transmission systems require wide-band receivers that are capable of receiving the optical signal and providing an electrical signal output. Transimpedance amplifiers are widely used in optical receiver applications as preamplifiers for converting received optical signals into an electrical signal output. However, the bandwidth performance of conventional transimpedance amplifier circuits is limited to a fraction of the bandwidth of the embedded transistors.
To facilitate the discussion that follows, it is important to define two terms of art with regard to bandwidth performance. The first is the frequency where the signal response of the circuit or device drops by 3 dB below the peak response; it is called the -3 dB bandwidth and is denoted as f-3dB. The second is the frequency where the circuit or device produces no gain (0 dB); it is called the unity gain cut-off frequency and is denoted by ft. For conventional transimpedance amplifiers, the f-3dB is determined by the dominant pole defined by the relationship ##EQU1## where R is the value of the feedback resistance and C is the value of the input capacitance at the active transistor's gate (or base). The f-3dB cannot be any larger than the ft of the embedded transistors and, in application, is usually lower than ft by at least a factor A (the open loop voltage gain). Therefore, to improve the bandwidth capability of conventional transimpedance amplifiers, developers have had to try to minimize R or C, or improve the technology of the embedded transistors used in the circuit. However, reducing the value of R increases the circuits susceptibility to external noise and reduces the gain in the circuit, thereby minimizing the effectiveness of the circuit's intended function. The value of the input gate capacitance (C) is a characteristic of the transistors used and whether the input source is capacitively coupled to the transistor gate. In conventional discrete component transimpedance amplifiers, capacitance coupling is used to protect the gate of FET transistors and the base of bipolar transistors from electro-static discharge (ESD). Therefore any effort to reduce C by employing a direct coupling design raises problems with circuit reliability. Otherwise, since the value of C is a function of the transistors used, to improve bandwidth by minimizing C requires changing or improving the transistor technology employed.
One approach to improve the bandwidth performance of transimpedance amplifiers has been to build monolithic transimpedance devices. Such devices eliminate the need for capacitance coupling thereby reducing the value of C. In addition, monolithic devices also minimize the interconnection parasitics, which minimization can improved bandwidth performance. (see Meyer, Blauschild, "A Wide-band Low-noise Monolithic Transimpedance Amplifier", IEEE Journal of Solid State Circuits, Vol SC-21, No. 4, Aug. 1986). Another approach has been to improve the bandwidth of the underlying device technology in a monolithic structure resulting in another step in improved bandwidth performance. One example of such a devices is a single ended high performance transimpedance amplifier using InAlAs/InGaAs heterostructures as shown by Chang et al. ("A 3 GHz Transimpedance OEIC Receiver for 1.3-1.55 .mu.-m Fiber-Optic Systems", G-K. Chang, W. P. Honig, J. L. Gimlett R. Bhat, C. K. Nguyen, G. Sasaki, and J. C. Yound, IEEE Photonics Letter, Vol. 2, No. 3, March 1990). However, in these examples of prior art the f-3 dB is still significantly lower than the ft of the embedded transistors.
In view of the foregoing, it is an objective of our invention to provide a circuit structure for transimpedance amplifiers that doesn't appreciably limit the bandwidth of the amplifier to below that of the embedded transistors. It is further an object of our invention to provide for a transimpedance amplifier circuit structure with improved noise immunity performance. It is also an object of our invention to obviate the reliability problems caused by direct coupling of the input source to the gate (or base) of the transistor.